Technology complying with the IEEE standard 1149.1 developed by the Joint Test Action Group (JTAG) has been used to successfully test board-level interconnects between devices (e.g., integrated circuits). The IEEE standard 1149.1 (hereinafter referred to as the “JTAG standard” or “JTAG”) specifies sufficient test coverage of faults for direct current-coupled (DC-coupled) interconnects only. A DC-coupled interconnect is a signal path having only wires and series resistances. A DC-coupled interconnect can pass both DC and AC components of a signal. The IEEE standard 1149.6 developed by JTAG is an extension of the JTAG standard that specifies test coverage of faults for AC-coupled interconnects. An AC-coupled interconnect is a signal path having a series capacitance that blocks DC component of a signal and only passes the AC component of the signal.
Integrated circuits (ICs) often include high-speed transceivers that are AC-coupled to board-level interconnects. For example, transceivers can be coupled by a differential signal path for low-voltage differential signaling (LVDS). A transceiver complying with the IEEE standard 1149.6 (hereinafter “AC-JTAG standard” or “AC-JTAG”) includes rest logic that can be used to test the structural correctness of the AC-coupled interconnect. The test logic in a transmitter modulates DC test data onto a time-varying AC waveform that can pass through the AC interconnect. The test logic in a receiver receives the AC waveform from the AC interconnect and recovers the DC test data. A transmitter compliant with AC-JTAG can designed to operate in mission mode (normal operating mode) or test mode. The transmitter should be designed so that the additional circuitry required to implement the test mode does not deleteriously affect the core logic circuitry that implements the mission mode.